1. Technical Field
This invention relates to fiber optic interferometers that incorporate optical fiber stretchers, and more particularly to fiber optic interferometers that incorporate optical fiber stretchers which stretch two or more fibers wound around them by the same amount.
2. Description of the Related Art
Optical fiber can be used to generate variable optical delays by controlling the length of a fiber loop. For example, multiple turns of optical fiber can be wound around a cylindrical piezo-electric (PZT) actuator under a sufficient tension which assures that the optical fiber never goes limp during operation. This forms a fiber optical delay device when an electrical voltage is applied to the cylindrical PZT actuator to cause the diameter of the cylindrical PZT actuator to change. This change leads to a change in the circumference of the cylindrical PZT actuator and thus changes the extent of stretching on the fiber loop, and thus the length of the optical fiber wound around the actuator. This design of an optical fiber stretcher can be used to achieve fast delay variations as was described by Tearney et al. in an article entitled “Rapid acquisition of a in vivo biological images by use of optical coherence tomography,” Optics Letters, Vol. 21, pp. 1408-1410 (1996). The response speed of such a PZT-based fiber delay device can be fast, e.g., on the order of kHz or tens of kHz. Applications of these types of optical fiber stretchers are described in Bush et al. in an article entitled “All-fiber optic coherence domain interferometric techniques”, Proc. SPIE, Vol. 4204, pp. 71-80 (2001).
Other examples of fiber optic delay lines are described in U.S. Pat. No. 5,835,642 by V. M. Gelikonov et al. entitled “Optical Fiber Interferometer and Piezoelectric Modulator” issued on Nov. 10, 1998, which describes a variable fiber optic delay line formed of a piezoelectric plate with electrodes on both of its surfaces. (“Gelikonov '642” subsequently herein.) In this delay line an optical fiber is coiled as a spiral and adhered to the piezoelectric plate on one or both surfaces of the plate. Applying an electric field to the piezoelectric plate results in a change in the diameter of the plate and hence the length of the optical fiber adhered to the plate. U.S. Pat. No. 5,867,268 by V. M. Gelikonov et al. entitled “Optical Fiber Interferometer With PZT Scanning of Interferometer Arm Optical Length” issued on Feb. 2, 1999 describes interferometer configurations using the Piezoelectric Modulators which are described in Gelikonov '642.
U.S. Pat. No. 7,382,962 by S. Y. Xiaotian entitled “Fiber stretcher apparatus” issued on Jun. 3, 2008 describes various designs of fiber stretchers utilizing one or more linear actuators to stretch fiber loops to create variable optical delay lines (“Xiaotian '962” subsequently herein). The fiber stretcher device described by Xiaotian '962 includes a stretcher frame that has a frame slot with a slot opening at one end to separate the frame into two parts that are connected at the other end of the frame slot. A linear actuator that expands or contracts along a straight line in response to a control signal produces a linear change in the dimension of the actuator along the straight line, and can be positioned across the frame slot with one end fixed to one frame part and the other end fixed to the other frame part. The linear expansion or contraction of the linear actuator exerts a force across the frame slot causing the frame slot to expand or contract, acting like a spring. This design transforms a linear expansion or contraction of the actuator into a change in the circumferential length of the stretch frame which can be shaped in various shapes having smooth surfaces including circles, ellipses, squares, and rectangles with rounded corners and racetrack shapes. This mechanism can be used to stretch a fiber loop formed by winding optical fiber around the exterior surface of the stretcher frame multiple times under tension. U.S. Pat. No. 7,206,076 by T. Blalock issued on Apr. 17, 2007 entitled “Thickness Measurement of Moving Webs and Seal Integrity System Using Dual Interferometer” (“Blalock '076” subsequently herein) also describes various designs of fiber optic stretchers.
Applications using optical fiber stretchers as fiber delay lines include low coherence interferometry (LCI) and optical coherence tomography (OCT). LCI and OCT have applications in many fields from medical imaging to glass manufacturing. LCI and OCT have been adapted to the non-contact measurement of physical properties of various materials including thickness, index of refraction measurement, surface profiles, and index of refraction profiles. LCI and OCT can also be used to measure distances between optical surfaces of a test structure. The LCI technique is based on using a light source with a short coherence length. The light is split between two arms or branches of an interferometer and then recombined and directed onto a detector. Interference occurs when the path lengths of light reflecting off of objects located in the two branches of the interferometer are equal to within the coherence length of the light from the source.
There are numerous known configurations of such interferometers, such as the Michelson, Mach-Zehnder, and Fizeau interferometers, and others described in the text, Principles of Optics: Electromagnetic Theory Of Propagation, Interference and Diffraction of Light, M. Born and E. Wolf, Cambridge University Press, Cambridge; New York, 1999, 7th ed. Another example of such an interferometer is described in U.S. Pat. No. 6,724,487 of M. A. Marcus et al. issued on Apr. 20, 2004 entitled, “Apparatus and method for measuring digital imager, package and wafer bow and deviation from flatness,” the disclosure of which is incorporated herein by reference. (“Marcus '487” subsequently herein.)
The interferometer disclosed therein by Marcus '487 as shown in FIG. 9 of Marcus '487 is based on the use of piezo fiber stretching technology as the means of changing the optical path-length in the two arms of the interferometer. A narrow beam of low-coherence light is directed onto the surface of the device under test. It is common to focus the beam inside or in proximity to the device under test. The reflected light from all of the optical interfaces of the device under test, which the beam traverses, is then combined with light from a coherent light source of a distinct wavelength using a wavelength division multiplexer (WDM). The combined light passes through a 50/50 fiber coupler into a pair of single mode optical fibers that are coiled around a pair of piezoelectric cylinders which make up the two arms of the fiber Michelson interferometer. Voltages are applied to the piezoelectric cylinders in a push-pull mode to alternately change the optical path length of light being transmitted along the optical fibers wound around the cylinders. Reflectors are located at the ends of the optical fibers after the coils to send the light back through the fiber coils. Interfering light returning from the interferometer arms is sent to separate low coherence and coherent light detectors. The coherent light interferometer provides a built in distance scale and is used to trigger data acquisition from the low coherence source at constant distance intervals. The low coherence interferometer is used to determine the optical distances between the interfaces in the device under test. The physical distances are obtained by dividing the optical distances by the group refractive indices of the material which makes up the space between the interfaces.
Other designs of dual interferometers in which a coherent light source is used together with a low coherence light source are described in U.S. Pat. No. 5,596,409 issued on Jan. 21, 1997 entitled “Associated Dual Interferometric Measurement Method for Determining a Physical property of an Object” by M. A. Marcus and S. Gross (“Marcus '409” subsequently herein), U.S. Pat. No. 5,659,392 issued on Aug. 19, 1997 entitled “Associated Dual Interferometric Measurement Apparatus for Determining a Physical property of an Object” by M. A. Marcus et al. (“Marcus '392” subsequently herein), “Blalock '076,” and U.S. Pat. No. 6,847,453 issued on Jan. 25, 2005 entitled “All Fiber Autocorrelator” by I. J. Bush. “Marcus '409” and “Marcus '392” describe how to use the coherent light interferometer as a distance scale by sampling at zero crossings of the coherent light interferogram and using the zero crossings to sample the low coherence light interferogram at constant distance intervals. This approach is called distance based sampling as opposed to the traditional approach of time based sampling.
In previous designs of dual interferometers using optical fiber stretchers as optical delay lines, the coherence source and the low coherence source operate at different wavelengths. The coherent light and low coherence light are combined using a WDM and are sent down a single fiber which changes path length while being transmitted along the fiber delay line. Optical fibers undergo dispersion effects when stressed and strained and different wavelengths of light have different coefficients of dispersion as a function of temperature and stress. This can cause changes in the calibration of the distance scale of the interferometer as a function of optical fiber stretcher temperature which can result in measurement errors.
Cost of components in building interferometers is also an important parameter to manufacturers of such instruments. Anything that can be done to reduce the cost of manufacturing by eliminating expensive optical components is desirable.
The disclosures of all of the aforementioned patents are incorporated herein by reference. The disclosures of these patents notwithstanding, there remains an unmet need for a more precisely controlled fiber delay line measurement apparatus and method that eliminates temperature dependent differential dispersion effects to provide a more precise calibration. There is also a need for a more precisely controlled low coherence interferometer to provide improved measurement reproducibility on measurements of physical parameters of test objects at a lower cost.